This invention relates generally to rewritable optical discs and optical disc drives used for digital data storage, and more specifically to an improved system and method for writing data relative to a sinusoidally varying displacement of a groove structure on an optical disc.
For rewritable media, on which data can be appended to a partially recorded medium, and on which previously written data can be erased and overwritten, data formats commonly provide gaps in the data for accommodating angular speed variations between drives, and for accommodating write clock drift. Rewritable data formats also commonly provide clock synchronization patterns for adjusting the write clock frequency and phase. For example, magnetic discs and tapes are typically formatted into sectors, with each sector including a preamble having a pattern suitable for synchronizing a write clock, and with each sector including extra space at the end to allow for variations in media velocity. Synchronization patterns and data gaps reduce effective data capacity because they occupy space that could otherwise be occupied by user data.
In contrast, some proposed formats for rewritable Digital Versatile Discs (DVD) do not have clock synchronization fields in the data or extra space at the end of data fields. Instead, these DVD formats require spatial features on the disc, and these DVD formats require data marks and spaces to be positioned, when written, with sub-bit accuracy relative to the spatial features on the disc. Because data integrity requires precise timing based on location of data marks and spaces, there is a general need for writing data marks and spaces at precise positions, and to be able to verify the placement precision. In general, the beginning and end of data marks and spaces are defined by edges of a write clock. However, there are various signal path delays that may vary with time and temperature, and signal path delays that may vary from drive to drive. In addition, the impact of these signal path delays may vary depending on the speed at which the disc is rotating during writing. There is a further need for an ability to control and verify spatial placement precision of data marks and spaces, even with variable unknown signal path delays.
Some optical disc formats have a land and groove structure, with at least one sidewall of the groove having a sinusoidal radial displacement (called wobble). See, for example, U.S. Pat. No. 6,046,968 (Abramovitch). Groove wobble may be frequency modulated to encode time or address information, or groove wobble may be used as a timing reference to synchronize a write clock. Some optical disc formats provide spatial features, such as notches in groove sidewalls, that are used for index marks, sector addresses, or for additional phase control of a write clock. See, for example, U.S. Pat. No. 5,933,411 (Inui et al.), and U.S. Pat. No. 5,852,599 (Fuji). See also, for example, M. Yoshida et al., xe2x80x9c4.7 Gbyte Re-writable Disc System Based on DVD-R Systemxe2x80x9d, IEEE Transactions on Consumer Electronics, Nov. 1, 1999, v 45, n 4, pp 1270-1276 (Yoshida et al.).
Optical disc drives may be required to rewrite multiple media formats. In general, an optical drive may not be able to depend on the presence of notches or tabs or similar spatial features on the medium (other than wobble) for timing control. Accordingly, there is a need for an ability to control and verify spatial placement of data marks and spaces without requiring spatial features on the medium other than wobble.
The light received at the surface of the detector array is not uniform, but instead comprises interference patterns, resulting in an intensity distribution. Binary data are encoded as transitions between areas of contrasting reflectance, or by pits and lands that affect the phase (and interference patterns) of the reflected light. Rewritable optical disc media commonly use a phase change material in a recording layer. During writing, the phase change material becomes crystalline when heated to just below its melting point and then cooled at a relatively slow rate, and amorphous when heated above its melting point and then cooled quickly. Data marks and spaces are formed by using focused laser light to heat small areas of the phase change material to one of two levels, and then allowing the material to cool. Crystalline areas typically reflect more light than amorphous areas. In general, the definitions of marks and spaces are arbitrary. That is, marks may be crystalline and spaces amorphous, or vice versa, and in general, marks may be more reflective than spaces, or marks may be less reflective than spaces. In the present patent document, marks are assumed to be crystalline, spaces are assumed to be amorphous, and crystalline areas are assumed to be more reflective than amorphous areas.
FIG. 1 (prior art) is a block diagram showing some of the signal paths in an example optical disc drive, to illustrate examples of various signal delays. In many optical disc drives, a single optical detector is used to generate a data signal, a radial position error signal, a focus error signal, and a wobble signal. FIG. 1 illustrates various lumped path delays for an optical disc drive using one optical detector for multiple functions. In FIG. 1, a light spot 100 is focused onto a data layer of an optical disc. Light reflected from the disc passes through various optical components before being detected by an optical detector 104. In FIG. 1, optical path delays between the disc and the detector 104 are lumped as Delay 1 (102). As depicted in FIG. 1, the optical detector 104 is divided into four sections (A,B,C,D), with each section providing a separate signal. The sum of the four signals (A+B+C+D), with some electronic filtering and processing, is the Analog Read-Data signal (108). Analog Read-Data signal path delays, due to filtering and other electronic processing, are lumped as Delay 2 (106). A radial position error signal, called a Radial Push-Pull (RPP) signal (112), is derived by subtracting appropriate pairs of the quad detector signals, for example (A+D)xe2x88x92(B+C). For media with wobbled grooves, the wobble signal is a high frequency modulation of the relatively low frequency RPP signal. Wobble may be extracted as a separate signal by high pass filtering the RPP/wobble signal. Accordingly, in the present patent document, the wobble signal may be referred to as RPP/wobble, or just Wobble, with the understanding that the RPP signal and the Wobble signal are often combined. In FIG. 1, various electronic filtering and processing delays for the RPP/wobble signal (112) are lumped as Delay 3 (110). If the Wobble signal is used for synchronization of a write clock signal, the Wobble signal is typically received by a Phase-Locked Loop (PLL, 114). The output of the PLL is used for a Write-Clock (116). A latch 120 is used to synchronize edges of a Write-Data signal (118) to edges of the Write-Clock (116), generating a Write Intensity signal (122). A Laser Intensity circuit 128 is controlled either by the Write Intensity signal (122) or by a Read Intensity signal (124), and the Laser Intensity circuit then controls the intensity of a laser diode light source. In FIG. 1, signal path delays in driving the Laser Intensity circuit 128, as well as any optical path delays, are lumped as Delay 4 (130).
Typically, Delay 1 and Delay 4 are negligible. Delay 2, Delay 3, delay through the PLL 114, and delay through the latch 120, however, are significant, and all may vary with time and temperature, and all may vary from drive to drive. In particular, note that there is a significant variable path delay (Delay 3+PLL+latch) between the time of a zero-crossing of spatial wobble and the time of an edge of the Write Intensity signal (122) in response to the zero-crossing of spatial wobble. Note also that the relative effects of these delays may vary if the writing speeds are different between drives. For example, if a disc is partially written in a drive at 1xc3x97speed, and rewritten in a drive at 2xc3x97speed, the delays have a different effect for the 2xc3x97drive relative to the 1xc3x97drive.
Consider a specific numeric example. In a proposed drive, a particular mark length is designated as a longest permissible mark, and longest marks must be placed with a leading edge at a zero-crossing of spatial wobble. For one particular writing speed, the specified maximum time from a spatial wobble zero-crossing to a spatial leading edge of a longest mark is on the order of 500 picoseconds. In a typical drive, Delay 2 is on the order of 2.5xc2x11 nanoseconds. The total of Delay 3 plus delays in the PLL 114 and latch 120 is on the order of 5.5xc2x13 nanoseconds. Therefore, even at one writing speed, the signal delays are on the order of 5-10 times the required precision, and the variability in the signal delays is on the order of 2-6 times the required precision. If a leading edge of a new longest mark is to be precisely located relative to a zero-crossing of spatial wobble, the system must compensate for Delay 2, and Delay 3, and the delays in the PLL 114 and the latch 120 in the example system of FIG. 1, or similar delays in functionally similar circuitry in other variations of drive designs.
A repetitive reference signal is obtained from a spatial feature on the disc. A repetitive Write-Timing signal is derived from the repetitive reference signal. The Write-Timing signal is offset in phase (earlier) relative to the reference signal. The system compensates for signal path delay by using the Write-Timing signal for writing data, thereby writing data early relative to the reference signal. In a specific example embodiment, the reference signal is derived from spatial wobble.
A calibration signal is generated that combines information used to generate the reference signal and to generate the Analog Read-Data signal. The data portion of the calibration signal, and the reference portion of the calibration signal, are both subject to the same delay. A calibration circuit measures signal amplitude (voltage or current) at two predetermined times in the calibration signal, and compares the magnitudes of the measured amplitudes. The phase of the Write-Timing signal, relative to the reference signal, is then adjusted based on the relative magnitudes of measurements of the calibration signal. In an example embodiment, the calibration circuit adjusts the phase of the feedback signal for a PLL relative to the phase of the Wobble signal, and the feedback signal for the PLL is used as the Write-Timing signal.
During reading, in the example embodiment, a drive modulates (reduces or increases current with an impulse) the normally constant current supplied to the laser diode. Read intensity impulses are generated in pairs, at times relative to a edge of the Write-Timing signal, during reading of a long mark or space. As a result of calibrating during long marks and spaces, no data signal transitions are present between or near two associated intensity impulses. The resulting calibration signal shows read intensity impulses superimposed on wobble, before and after a peak of the wobble as detected in the calibration signal. The phase of the Write-Timing signal is adjusted until the two intensity impulses, as detected in the calibration signal, are the same amplitude (and therefore symmetrical in time relative to a peak of the wobble as detected in the calibration signal). When the intensity impulses in the calibration signal are equal magnitude, the pulses have been written symmetrically relative to a peak of spatial wobble. Then, by using the Write-Timing signal during writing, an edge of a data mark is written at a time that has been adjusted for multiple signal path delays.
During writing of amorphous areas, the laser intensity is normally constant. Accordingly, for calibration during writing of amorphous areas, the laser can be modulated just as for calibration during reading. During writing of crystalline areas, the laser intensity is normally modulated to reduce the temperature of the focussed spot. For calibration during writing of crystalline areas, the calibration circuit monitors the normal modulations of laser intensity. For either type of area, the calibration signal shows intensity modulations superimposed on wobble. The phase of the feedback signal for the PLL is adjusted until two measured amplitudes of the calibration signal are equal (and therefore symmetrical in time relative to a peak of wobble as detected in the calibration signal).
By use of the calibration signal to adjust the phase of the feedback signal for the PLL, and by use of the feedback signal for the PLL as a Write-Timing signal, the example optical disc drive compensates for signal path delays. Adjustment of the phase of the Write-Timing signal is made frequently, during reading and writing, without requiring spatial timing features on the medium other than wobble.